1. Field of the Invention
The present invention relates generally to a photolithographic apparatus, and more specifically, to a photolithographic apparatus for a photo-resist that contains a photoacid generator and a photobase generator.
2. Description of the Related Art
With the rapid development of the microelectronic industry, critical dimensions of a semiconductor device continue to shrink. The shrinkage of the critical dimensions of a device depends on exposure subsystems. On the other hand, the shrinkage is closely related to properties of a photo-resist. Thus, the choice of photo-resist properties associated with photolithography is important.
The advancement of photolithography promotes the steady improvement in performances of a photo-resist. A chemical amplification photo-resist has high sensitivity and strong capability to withstand dry corrosion to facilitate subsequent processes of semiconductor devices. The chemical amplification photo-resist thus has a broader application prospect in the semiconductor manufacturing field and gradually gains attention in the photolithographic process. It is believed that the chemically amplified photo-resist having steady processing properties will play an important role in the semiconductor industry.
The chemically amplified photo-resist generally contains three components: a matrix resin, an organic solvent, and a photoacid generator (PAG). After the chemically amplified photo-resist has been exposed or illuminated with light, the PAG absorbs energy and undergoes photolysis. Thus, free acid is generated, which results in an acid catalytic reaction such that the matrix resin in exposure region undergoes removal reaction of protecting groups or a cross-linking reaction between resin and cross linker, forming positive or negative latent images which are then subjected to development in a certain solvent to form exposure images. In addition, some chemically amplified photo-resists employ a photobase generator (PBG) instead of a photoacid generator. An alkaline catalytic reaction takes place with the help of photobase, which likewise results in that the matrix resin undergoes a removal reaction of protecting groups or a cross-linking reaction between resin and cross linker, forming positive or negative latent images.
However, the contrast of latent images will be degraded due to following factors: One factor is the diffusion of photoacid or photobase. The photoacid or photobase generated by illumination with a light of a first wavelength band gradually diffuses from a position of high mass concentration to a position of low mass concentration. In this way, the mass concentration distribution of photoacid or photobase departs from the optical image and degrades the contrast of latent images of photoacid or photobase. The second factor is photo diffraction. Theoretically, an optical image formed by means of a mask should be a simple binary image. That is, in the optical image, the light intensity of a part of the image where the light is sheltered by the mask is zero while the light intensity of the other part of the image where the light transmits through the mask is a constant. However, in practice, with the continuous shrinkage of the critical dimension for a semiconductor process, light diffraction effect becomes more severe, such that the part of the optical image that should have zero light intensity has a non-zero amount of light intensity. As a result, the contrast of the latent image of the photoacid is further degraded.
In the conventional photolithographic process, a method of restricting the diffusion length of photoacid or photobase is employed to enhance the contrast of latent images. However, the method is disadvantageous since it will make the removal reaction or the cross-linking reaction less efficient. Moreover, the conventional method does not prevent the degradation of contrast of latent images caused by diffraction.